am: Centralize process unfreeze logic in AMS

The `unfreezeTemporarily` method is now exposed directly by
`ActivityManagerService`. This refactors internal callers in
`BroadcastQueue` and `ContentProviderHelper` to use the new
API, providing a more consistent and centralized entry point
for managing temporary unfreezing of processes.

Bug: 441178013
Test: m services.core
Test: atest MockingOomAdjusterTests OomAdjusterTests
Test: atest FrameworksServicesTestsRavenwood_ProcessStateController
Test: atest BroadcastQueueTest
Flag: EXEMPT PURE_REFACTOR

Change-Id: I3da095b13e0848f99beb5de040c52592dd330a4b